This invention relates to preparation of supported catalysts and the supported catalysts prepared.
Supported catalysts are used to increase reaction rates over uncatalyzed reactions and/or obtain reaction products different from those produced in uncatalyzed reactions.
Solid catalysts react on their surface. Increased catalytic effect for a given amount of a catalyst can be accomplished by increasing the surface area of the catalytic material. One method to increase the surface area is to change the shape of the catalytic material. A more effective method of increasing the surface area of a catalyst is decreasing the size of the catalyst particles (and thereby increasing the number of catalyst particles). This method utilizes scaling factors. The surface area of a particle is proportional to the square of the particle's linear dimension (such as diameter). The amount or weight of the particle is proportional to the cube of the linear dimension. Therefore, the surface area of a given fixed amount of a catalytic material is proportional to the reciprocal of the linear dimension provided the shape of the particles remain constant.
Generally, smaller catalyst particles have a larger specific surface area thereby allowing the use of a smaller mass of catalyst for the same catalytic effect. However, small catalytic particles are expensive to remove from most systems. Therefore, the use of a catalyst support, which is large relative to the catalytic particles to which small catalytic particles are affixed, allows use of small particles of a catalytic material without creating the problem of removal of small catalytic particles.
The catalytic particles must be affixed to the surface of the catalyst support in order to have a catalytic effect. Many methods of affixing the catalytic particles to the surface of the catalyst support's surface are known. For example, U.S. Pat. No. 3,856,709 discusses the process of mashing soft particles onto a relatively hard substrate.
To increase the surface area of the catalyst support, without reducing the size of the support, a porous support is used. In order to use this increased surface area, the catalytic particles must be deposited on the inside of the pores of the catalyst support.
U.S. Pat. No. 2,107,611 discusses a process of spraying an acetone suspension of small catalytic particles onto support particles, then drying the acetone to leave catalytic particles on the support.
U.S. Pat. No. 2,818,350 discusses a process of vaporizing a vaporizable fluid of a suspension of solid alkali metal particles in a vaporizable fluid to attach the catalytic particles to the catalyst support.
U.S. Pat. No. 3,216,947 discusses a process of depositing a sodium metal on a catalyst support by high speed stirring (10,000 rpm) of sodium, a high boiling hydrocarbon and potassium at an elevated temperature.
U.S. Pat. No. 3,264,226 discusses a process of plasma arc spraying of catalytic material upon a catalyst support.
U.S. Pat. No. 4,255,285 discusses a process of atomizing a water solution of catalyst ions, applying the atomized solution to moving catalyst supports, and then drying and heat treating the wetted supports.
All of the aforementioned processes lack the ability to deposit catalytic particles deep into the pores of a catalyst support.
U.S. Pat. No. 4,305,844 discusses a "suspension method" for forming supported catalysts which involves mixing a suspension of silver oxide with the support. After the suspension is thus deposited on the support, the catalyst is activated. As pointed out in U.S. Pat. No. 4,305,844, this method produces a supported catalyst which readily loses catalytic particles. The process also has trouble depositing catalytic particles deep in the pores of the support.
Another method of depositing metals on supports is known as the "impregnation method". Generally, the support is impregnated or coated with a solution of the salt of the metal or metals to be employed. This is followed by drying and a subsequent reduction.
Although the reducing step can be a thermal reduction in the presence of air or an inert gas (U.S. Pat. No. 2,709,123), hydrogen and hydrazine (U.S. Pat. No. 3,575,888) have also been employed for the reduction. Reducing agents in the form of organic compounds have been added to the catalyst, either by simultaneous application to the support or subsequent addition to the impregnated metal salts. Also, reducing compounds have been known to be employed by incorporating them into the support prior to the addition of the catalytic component. While the solvent used to apply the reducing agent is frequently water, as in U.S. Pat. No. 3,702,259, organic solvents also have been employed, as for example, ethylene carbonate, dimethyl formamide and dimethyl sulfoxide, as solvents for the polyacrylonitrile in U.S. Pat. No. 3,892,679.
In U.S. Pat. No. 3,892,679, the use of non-aqueous solvents to precipitate a polymer from solution onto the support is indicated and is illustrated by the use of methanol and toluene. This method disadvantageously requires the preparation of a salt solution, impregnation of a support (if porous support is used) with the solution, drying of the impregnated support and reduction of the deposited salt. At times special care is needed to remove an undesirable counterion such as chloride. Some ions such as zinc are difficult to reduce and usually are present in the supported catalyst as zinc oxide or zinc (II). For example, the reduction potential of Zn.sup.+2 to Zn.degree. is -0.76 v and that of ZnO.sub.2 to Zn.degree. is -1.22 v.
It would be desirable to have a process which avoids the multiple steps of the "impregnation method" and the loose binding of the "suspension method", and yet has the ability to deposit metals deep into pores of porous catalyst supports. It would be even more desirable if such a process has the ability to deposit metals which are hard to reduce onto catalyst supports. Additionally, it would be desirable if the deposited metal can be converted by a simple procedure to a deposited catalytic metal.